A so-called compact microphone is practically a capacitor microphone. Regarding to a goose-neck type capacitor microphone which is seen at a conference room or the like, a microphone of which a microphone capsule can be exchanged is in existence in order to change the microphone characteristics according to the surroundings. Referring to FIG. 5, one of the examples will be described.
A capacitor microphone includes a capsule support 10 which is mounted on the top end of a supporting pipe P and a microphone capsule 20 which is removably connected to the capsule support 10 with a screw latch. The supporting pipe P generally stands on a desk through a microphone stand (not shown) and a part of the pipe P uses a flexible shaft.
The microphone capsule 20 has a cylindrical chassis 21 formed with aluminum or the like. It is not shown that a vibrating plate and a fixed electrode are faced to each other and disposed through an insulating spacer in the chassis 21, the vibrating plate strained and fixed on a supporting ring, the fixed electrode formed with an electret board or the like and supported on an insulating pedestal.
The back side of the chassis 21 is closed with a rear cover 22. A contact pin 23 connected to the fixed electrode protrudes from the rear cover 22. A connecting screw 24 which has an internal screw 241 on the inner surface thereof for connecting to the capsule support 10 is fixed on the back side of the chassis 21 to be electrically conducted to the chassis 21.
The capsule support 10 has a cylindrical chassis 11 formed with brass or the like. A circuit board 12 is disposed in the chassis 11 as to close the inside of the chassis. An impedance converter (FET: Field Effect Transistor) 13 is mounted on the circuit board 12. One end of an electric cable 17 which is passed and extracted through the supporting pipe P is soldered on the lower surface of the circuit board 12.
A piece of a contacting terminal 14 which is bent in a substantial V-shape and which is formed with a plate spring is soldered on the gate terminal of the impedance converter 13 to elastically contact to the contact pin 23. The circuit board 12 is fixed in the chassis 11 with a fixing ring 15.
That is, the outer circumference of the fixing ring 15 has an external screw 151 and on the other hand, a step 111 receiving the circuit board 12 and an internal screw 112 screwing with the external screw 151 are formed in the chassis 11. The circuit board 12 is pressed and fixed to the step 111 by screwing the fixing ring 15 to the internal screw 112.
The length of thread engagement of the fixing ring 15 with the internal screw 112 is a substantially lower-half of length of the external screw 151. The remaining upper-half of the external screw 151 is screwed with the internal screw 241 of the connecting screw 24 to connect to the microphone capsule 20. After the circuit board 12 has been fixed, a bulking agent 16 of silicon resin or the like is filled in a recess between the impedance converter 13 and the fixing ring 15.
The microphone capsule 20 and the capsule support 10 are mechanically connected by screwing the internal screw 241 of the connecting screw 24 to the external screw 151 of the fixing ring 15 so that the contact pin 23 elastically contacts and electrically connects to the contacting terminal 14. The chassis 21 of the microphone capsule 20 and the chassis 11 of the capsule support 10 are also electrically conducted through the fixing ring 15.
According to the connecting structure described above, the microphone capsule 20 can be easily attached to and removed from the capsule support 10 only by turning the microphone capsule. However, since the bulking agent 16 of silicon resin or the like is filled in the recess between the impedance converter 13 and the fixing ring 15, acoustic or environmental problems and problems in assembly or in maintenance, which will be described hereinafter, have been generated.
The recess between the impedance converter 13 and the fixing ring 15 increases the volume of the rear air chamber of the microphone capsule 20. The larger volume of the rear air chamber becomes a factor which generates an acoustic resonance and the resonance exerts bad influences upon a frequency response or a directional characteristic of the microphone capsule 20.
Therefore, in order to prevent things described above, the bulking agent 16 of silicon resin or the like is filled in the recess between the impedance converter 13 and the fixing ring 15, however, it is difficult to maintain the volume of the bulking agent. Since each of the microphones has each different volume of the rear air chamber, the variation of the air volume exerts subtle influences upon the frequency response or the directional characteristic of the microphone capsule 20.
Further, a dry process needs to be added after the bulking agent is filled and it takes more time to produce the microphones due to the additional dry process. The process reduces the productivity of the microphones. Further, a problem in a maintenance work occurs as follows. For example, the bulking agent 16 may be damaged when the circuit board 12 is removed. In this case, the bulking agent needs to be removed and the filling work of the bulking agent should be repeated.
Another problem besides that relating to the bulking agent may happen. Since the contacting terminal 14 is soldered to the gate terminal of the impedance converter 13 in the prior art, the assembling work including the soldering work is complicated so that the productivity of the microphone is reduced.
When a solder with lead is used, the care for the environment needs. Lead-free solder can be used, however, the lead-free solder needs higher heating temperature and is more expensive than the solder with lead so that the lead-free solder cannot be employed.